Giving two LAN-to-WAN connected routers same IP address during setup

Installing and Using OpenWrt Network and Wireless Configuration
1

Can I work on setting up Router B in this configuration with a view to Router B's replacing Router A in position 1 when the setup completes? (Notice the same LAN-side IP address for both routers.)

routers

I know that I cannot use Router B as pictured because a computer plugged into its LAN and receiving DHCP service from it could potentially see two hosts with the same IP address (one plugged into Router A and the other to Router B).

So my question is only on setting up Router B. B would have nothing under it (on its LAN-side) while being set up. It would have a redirect rule like:

config redirect
	option name 'OUT_IN'
	option target 'DNAT'
	option src 'wan'
	option src_dport '22001'
	list proto 'tcp'
	option dest 'lan'
	option dest_ip '192.168.0.1'
	option dest_port '22'
	option enabled '1'

So, from Computer I could use something like ssh -p 22001 -D 1081 -q root@192.168.0.10 for both Luci and SSH access, flash Router B with firmware image, and also upload (e.g. rsync) config files.

When all setup is done, Router B would replace Router A in position 1.

The only alternative I can think of is to put Router B and a computer in their own LAN away from my everyday LAN with access to the Internet (that is, if I don't want any interruption in the use of my everyday LAN).

ADDED LATER
I will actually try the above and see (a) whether it works and (b) whether it gives me any unforeseen trouble. Remember "working" just means being able to SSH from Computer via the redirect rule to Router B and edit config files. B does not have to work as a router (until it takes position 1). I don't feel anybody has told me that this would NOT work.

2

If you are not going to connect anything to the LAN side of router B, why do you need to configure it on the same IP range? Give it another IP address, do all the configuration, then reconfigure it and reboot.

3

It's because Router A has all kinds of configs that have 192.168.0.x in it (static leases, redirects) and Router B will later step into A's place.

Suppose during setup B has 10.0.0.1 (on LAN side).

Can I still upload to B config files that have all kinds 192.168.0.x in it without messing things up? (Wouldn't Router B think: Wait a minute. I am 10.0.0.1. How am I supposed to redirect to 192.168.0.100?)

Well, suppose that's not a problem.

But there would come a final moment where I have to do one of these two things:

  • (i) Kick A from position 1 > put B in position 1 > let Computer receive a random IP address in 10.0.0.x > SSH to B and change its IP to the desired 192.168.0.1 > reboot everything or

  • (ii) leave B in position 2 > there change its IP to 192.168.0.1 > kick A from position 1 > put B in position 1.

I would prefer (ii) because I don't have to mess with Computer receiving an IP address in 10.0.0.x. But if (ii) is OK, which means it is OK for A and B both to have 192.168.0.1 for that final step, the reasoning goes, why not just do it that way for the whole setup process?

4

If router B is configured with two interfaces on the same IP range, it's going to be way more confused...

5

I may be getting out of my depth here, but if I understand the quoted bit:

Router A (in present use) has only one LAN interface (beside one WAN and one WAN6). And Basically I want to replicate A in B (same settings), kick A out, and put B in.

So "configuring router B with two interfaces on the same IP range" would not describe what I am trying to accomplish (I hope).

During setup, router B will have just one LAN interface. Only, it's IP will be the same as that of A's LAN interface.

But I thought that would be OK because router B doesn't have to "work" during setup. It only has to receive Luci and SSH traffic from Computer for flashing and uploading of config files through WAN port 22001 (per redirect rule above).

Am I totally wrong?

Sorry, I figured it out. You mean that router B's having LAN and WAN (thus 2 interfaces) in the same IP range will give me trouble?

If so, the only way to replicate Router A's settings in B is to put B (during setup) in a separate LAN, e.g. made of just B and a computer with the computer uploading A's setting to B?

6

Just don't connect WAN on router B during setup and you're good. If you need to install packages, you can do so before installing OpenWrt by using the firmware-selector and integrating all packages right from the start (this also saves you space).

So yes, that should work.